A case-control study in Rio de Janeiro in 1994 included 552 patients with a positive culture for Mycobacterium tuberculosis and reported a 1.8% resistance rate to rifampicin and isoniazid. Prior treatment for tuberculosis (the exposure factor) was associated with resistance. The binomial proportion was used to test the hypothesis of no dependence between the observed associations based on prior exposure to the drugs. This test showed a greater than expected dependence between resistances to rifampicin and isoniazid.